Fluid Level Detector

ABSTRACT

A fluid detector for determining a presence of a fluid within a container includes a piezoelectric element that outputs a first ultrasonic signal in response to an input electrical signal and a lens with an upper portion and a lower portion. The piezoelectric element is coupled to the upper portion of the lens so that, when the lens is disposed adjacent the outer surface of the wall such that the lens is intermediate the piezoelectric element and the wall, the lens focuses the first ultrasonic signal toward the wall so that the first ultrasonic signal enters the wall.

CLAIM OF PRIORITY

This application is a continuation of U.S. patent application Ser. No.11/032,976, filed on Jan. 10, 2005, entitled “Fluid Level Detector,”which is hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present disclosure relates generally to fluid level detectors foruse with various sized containers. More particularly, the presentdisclosure relates to a fluid level detector including a piezoelectricelement that may be used to determine the presence or absence of a fluidwithin the container to which the fluid level detector is attached.

BACKGROUND OF THE INVENTION

The use of piezoelectric materials in fluid level sensors is known. Anexisting design includes two piezoelectric sensor elements mountedopposite each other on the inside of a container. The sensor elementsare both mounted at the level of interest. A first sensor elementfunctions as a transmitter and is electrically excited with a voltagepulse or continuous frequency such that it transmits an acoustic signal.The second sensor element functions as a receiver of the transmittedacoustic signal. When both sensor elements are immersed in a fluid, theacoustic signal generated by the first sensor propagates through thefluid and is detected by the second sensor element, thereby indicatingthe presence of fluid at the level of the sensor elements. In thepresence of air, the acoustic signal is not detected by the secondsensor element, indicating that fluid is not present at the level ofinterest.

As noted, existing fluid level sensors often require intimate contactbetween the sensor elements and the fluid being detected. As well,because the sensor elements are typically mounted inside the container,the structural integrity of the container must be breached to installthe sensor elements. As such, the container must usually be empty, or atleast not have fluids at or above the level of interest, when the sensorelements are being installed.

SUMMARY OF THE INVENTION

The present disclosure recognizes and addresses the foregoingconsiderations, and others, of prior art constructions and methods.Accordingly, it is an object of the present disclosure to provide animproved fluid level detector.

This and other objects are achieved by a transducer for use in a fluiddetector for determining a presence of a fluid within a container, thecontainer having a wall with an outer surface and an inner surface. Thetransducer includes a piezoelectric element that outputs an ultrasonicsignal in response to an input electrical signal and a lens with anupper portion and a lower portion. The piezoelectric element is coupledto the upper portion of the lens so that, when the lens is disposedadjacent the outer surface of the wall such that the lens isintermediate the piezoelectric element and the wall, the lens focusesthe ultrasonic signal toward the wall.

Another embodiment of the present disclosure includes a fluid detectorfor determining a presence of a fluid within a container, the containerhaving a wall with an outer surface and an inner surface. The fluiddetector includes a piezoelectric element that outputs a firstultrasonic signal in response to an input electrical signal and a lenswith an upper portion and a lower portion. The piezoelectric element iscoupled to the upper portion of the lens so that, when the lens isdisposed adjacent the outer surface of the wall such that the lens isintermediate the piezoelectric element and the wall, the lens focusesthe first ultrasonic signal toward the wall so that the first ultrasonicsignal enters the wall. The fluid detector further includes anultrasonic detector that, when disposed in a predetermined positionadjacent the outer surface of the wall, receives a second ultrasonicsignal from the wall that results from the first ultrasonic signal andthat is affected in a predetermined manner by presence or absence offluid at the inner surface of the wall. The fluid detector furtherincludes an ultrasonic detector that, when disposed in a predeterminedposition adjacent the outer surface of the wall, receives a secondultrasonic signal from the wall that results from the first ultrasonicsignal and that is affected in a predetermined manner by presence orabsence of fluid at the inner surface of the wall. The ultrasonicdetector generates an output electrical signal corresponding to thesecond ultrasonic signal.

Yet another embodiment of the present disclosure includes a fluiddetector for determining a presence of a fluid within a container, thecontainer having a wall with an outer surface and an inner surface. Thefluid detector includes a polymer piezoelectric element that outputs afirst ultrasonic signal in response to an input electrical signal and apolymer lens with an upper portion and a lower portion. Thepiezoelectric element is coupled to the upper portion of the lens sothat, when the lens is disposed adjacent the outer surface of the wallsuch that the lens is intermediate the piezoelectric element and thewall, the lens focuses the first ultrasonic signal toward the wall sothat the first ultrasonic signal enters the wall. The fluid detectorfurther includes an ultrasonic detector that, when disposed in apredetermined position adjacent the outer surface of the wall, receivesa second ultrasonic signal from the wall that results from the firstultrasonic signal and that is affected in a predetermined manner bypresence or absence of fluid at the inner surface of the wall. Theultrasonic detector generates an output electrical signal correspondingto the second ultrasonic signal.

A further embodiment of the present disclosure includes a fluid detectorfor determining a presence of a fluid within a container, the containerhaving a wall with an outer surface and an inner surface. The fluiddetector includes a housing, an electrical signal source, and apiezoelectric element disposed in the housing that outputs a firstultrasonic signal in response to an input electrical signal provided bythe signal source. A spring is included having an electricallyconductive element, wherein the spring is electrically coupled betweenthe electrical signal source and the piezoelectric element so that thespring conducts the input electrical signal between the electricalsignal source and the piezoelectric element. The spring is also disposedin the housing in operative communication with the piezoelectric elementso that, when the housing is disposed adjacent the outer surface of thewall, the spring biases the piezoelectric element operatively toward theouter surface of the wall so that the first ultrasonic signal isdirected to the wall. The fluid detector further includes an ultrasonicdetector that, when disposed in a predetermined position adjacent theouter surface of the wall, receives a second ultrasonic signal from thewall that results from the first ultrasonic signal and that is affectedin a predetermined manner by presence or absence of fluid at the innersurface of the wall. The ultrasonic detector generates an outputelectrical signal corresponding to the second ultrasonic signal.

Another embodiment of the present disclosure includes a fluid detectorfor determining a presence of a fluid within a container, the containerhaving a wall with an outer surface and an inner surface. The fluiddetector includes a housing, an electrical signal source, and apiezoelectric film element disposed in the housing that outputs a firstultrasonic signal in response to an input electrical signal provided bythe electrical signal source. The piezoelectric film element has a topside and a bottom side. A lens with an upper portion and a lower portionis included, and the upper portion of the lens is coupled to the bottomside of the piezoelectric film element. A first spring with anelectrically conductive element is electrically coupled between theelectrical signal source and the top side of the piezoelectric filmelement so that the first spring conducts the input electrical signalbetween the electrical signal source and the piezoelectric film element.The first spring is also disposed in the housing in operativecommunication with the piezoelectric film element so that, when thehousing is disposed adjacent the outer surface of the wall, the firstspring biases the piezoelectric film element operatively toward theouter surface of the wall so that the first ultrasonic signal isdirected to the wall. A second spring having an electrically conductiveelement is electrically coupled to the bottom side of the piezoelectricfilm element. The fluid detector also includes an ultrasonic detectorthat, when disposed in a predetermined position adjacent the outersurface of the wall, receives a second ultrasonic signal from the wallthat results from the first ultrasonic signal and that is affected in apredetermined manner by presence or absence of fluid at the innersurface of the wall. The ultrasonic detector generates an outputelectrical signal corresponding to the second ultrasonic signal.

Yet another embodiment of the present disclosure includes a fluiddetector for determining a presence of a fluid within a container, thecontainer having a wall with an outer surface and an inner surface. Thefluid detector includes a housing having a base and a piezoelectricelement disposed in the housing that outputs a first ultrasonic signalin response to an input electrical signal. An adhesive layer is disposedbetween the base and the outer surface of the wall, thereby securing thehousing to the wall when the housing is disposed in a position adjacentthe wall so that the first ultrasonic signal is directed to the wall.The fluid detector also includes an ultrasonic detector that, whendisposed in a predetermined position adjacent the outer surface of thewall, receives a second ultrasonic signal from the wall that resultsfrom the first ultrasonic signal and that is affected in a predeterminedmanner by presence or absence of fluid at the inner surface of the wall.The ultrasonic detector generates an output electrical signalcorresponding to the second ultrasonic signal.

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the fluid leveldetector and, together with the description, serve to explain theprinciples of the fluid level detector.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the fluid level detector, includingthe best mode thereof to one of ordinary skill in the art, is set forthmore particularly in the remainder of the specification, which makesreference to the accompanying figures, in which;

FIG. 1 is an exploded, perspective view of a fluid level detector inaccordance with an embodiment of the present disclosure;

FIG. 2 is a side view of the assembled fluid level detector as shown inFIG. 1;

FIG. 3 is a bottom view of the assembled fluid level detector as shownin FIG. 1;

FIG. 4A is an exploded, perspective view of a sensor assembly of thelevel detector as shown in FIG. 1;

FIG. 4B is a top perspective view of the assembled sensor assembly asshown in FIG. 4A;

FIG. 5A is a side, cross-sectional view of the sensor assembly as shownin FIG. 4B, taken along line 5A-5A;

FIG. 5B is a side, cross-sectional view of the sensor assembly as shownin FIG. 4B, taken along line 5B-5B;

FIG. 6 is a detailed, partial cross-sectional view of the sensorassembly as shown in FIG. 5B;

FIG. 7A is a side, cross-sectional view of the fluid level detector asshown in FIG. 3, along line 7A-7A;

FIG. 7B is a side, cross-sectional view of the fluid level detector asshown in FIG. 7A, positioned adjacent a container wall;

FIG. 8 is an electrical schematic of an electronic module for use with afluid level detector as shown in FIG. 1;

FIG. 9 is an electrical schematic of an excitation circuit for use in anelectronic module as shown in FIG. 8; and

FIGS. 10A and 10B are graphical representations of signals processed bythe electronic module as shown in FIGS. 8 and 9.

Repeat use of reference characters in the present specification anddrawings is intended to represent same or analogous features or elementsof the fluid level detector according to the disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to presently preferred embodimentsof the fluid level detector, one or more examples of which areillustrated in the accompanying drawings. Each example is provided byway of explanation, not limitation, of the fluid level detector. Infact, it will be apparent to those skilled in the art that modificationsand variations can be made in the present fluid level detector withoutdeparting from the scope or spirit thereof. For instance, featuresillustrated or described as part of one embodiment may be used onanother embodiment to yield a still further embodiment. Thus, it isintended that the present disclosure covers such modifications andvariations as come within the scope of the appended claims and theirequivalence.

Referring now to FIG. 1, a fluid level detector 100 includes a bottomhousing 110, a top housing 130, a wiring harness 140, and a sensorassembly 160 (FIGS. 4A and 4B). As described in more detail below,sensor assembly 160 comprises an ultrasonic transducer. Bottom housing110 includes a generally cylindrical upper wall 112 and a disc-shapedbase 118. Upper wall 112 defines a generally cylindrical central bore114 disposed about the longitudinal center axis of bottom housing 110.Upper wall 112 further includes an annular groove 116 extending inwardlyfrom its outer surface and an annular lip 117 extending outwardly fromits outer surface. Base 118 defines an aperture 120 (FIG. 7A) that it isin communication with the central bore 114 and a bottom surface 122 thatis configured for abutment with a container wall 104 (FIG. 7B). Aperture120 is transverse to the longitudinal center axis of central bore 114and has a diameter that is smaller than the diameter of central bore114.

Top housing 130 includes a top portion 136 and a substantiallycylindrical wall 132 extending downwardly therefrom. Cylindrical wall132 is configured to slidably receive upper wall 112 of bottom housing110. An annular groove 133 (FIGS. 7A and 7B) extends outwardly into thecylindrical wall 132 of top housing 130 and is located and configuredsuch that when upper wall 112 of bottom housing 110 is adequatelyinserted into cylindrical wall 132, annular lip 117 on upper wall 112 isfirmly seated in annular groove 133, thereby securing bottom housing 110and top housing 130 together. A wiring harness receptacle 134 forslidably receiving wiring harness 140 is formed in cylindrical wall 132.Preferably, both bottom housing 110 and top housing 130 are formed frommolded polymers such as, but not limited to acrylonitrile butadienestyrene. However, it should be appreciated that any suitable materialcould be utilized.

Prior to assembling bottom housing 110 and top housing 130, an O-ring150 is positioned in annular groove 116 of upper wall 112. O-ring 150serves to prevent dirt, debris, and humidity from entering fluid leveldetector 100 after bottom and top housings 110 and 130 are assembled.

Referring now to FIGS. 4A and 4B, transducer 160 includes a lens 162, apiezoelectric element 170, a conductive sleeve 180, an insulative disc186, a tab contact 190, and a conductive pad 194. Lens 162 is generallycylindrical in shape and includes an annular ledge 168 disposed betweenupper and lower portions of lens 162 and extending radially from themain body of the lens. The upper portion of lens 162 includes a domedsurface 164 having a conductive layer 163 (FIG. 6) formed thereon. Thelower portion of lens 162 defines a contact face 166 that abuts acontainer wall 104 (FIG. 7B) when fluid level detector 100 is installedfor operation. Preferably, lens 162 is formed of a material exhibitinglow acoustic loss and an acoustic impedance similar to that ofpiezoelectric element 170 and container wall 104. Preferably, lens 162is constructed of polystyrene, REXOLITE, or other similar materials. Aswell, an example of a suitable material for conductive layer 163 iscopper, although it should be appreciated that many materials exhibitsuitable electrical conductivity and could be utilized.

Piezoelectric element 170 is preferably a flexible piezoelectric filmelement (preferably a suitably processed polyvinylidene fluoridecopolymer (PVDF)) having a top surface 172 and a bottom surface 174. Afirst electrode layer 176 (FIG. 6) and a second electrode layer 178(FIG. 6) are formed on top and bottom surfaces 172 and 174,respectively. First and second electrode layers 176 and 178 are isolatedelectrically from each other by piezoelectric film element 170. Becausepiezoelectric film element 170 is flexible, it conforms to the shape ofdomed surface 164 of lens 162 when secured thereto.

Conductive sleeve 180 is substantially cylindrical and includes aninwardly depending lip 182 at the top end and an edge 184 at the bottomend that is configured to abut annular ledge 168 of lens 162 whentransducer 160 is assembled. Further, the inner diameter of conductivesleeve 180 is slightly larger than the outer diameter of the upperportion of lens 162 such that lens 162 is partially insertable intoconductive sleeve 180. Preferably, conductive sleeve 180 is formed ofstainless steel, or other similarly conductive materials.

Insulative disc 186 defines a central aperture 188 that is configured toreceive a portion of tab contact 190. The outer diameter of insulateddisc 186 is slightly less than the inner diameter of conductive sleeve180 such that insulative disc 186 can be disposed inside conductivesleeve 180, adjacent inwardly depending lip 182. Disc 186 is formed ofany material suitable for the purpose of insulating conductive sleeve180 from tab contact 190, preferably a polymer such as, but not limitedto, acrylonitrile butadiene styrene, for example marketed under the nameCYOLAC MG94 by GE Plastics.

Tab contact 190 includes a portion that is insertable into centralaperture 188 of disc 186 and a planar surface 192 having a diametergreater than that of central aperture 188. As such, planar surface 192prevents the passage of tab contact 190 through central aperture 188.Preferably, tab contact 190 is formed of nickel plated brass. However,other similarly electrically conductive materials are acceptable. Aconductive pad 194 is comprised of foam with a nickel plating and has anouter diameter such that it is at least partially insertable intocentral aperture 188 of insulative disc 186. Although plated, conductivepad 194 remains pliant and thereby facilitates electrical contact ofconductive pad 194 with both piezoelectric film element 170 and tabcontact 190. After transducer 160 is assembled, piezoelectric filmelement 170 is disposed between domed surface 164 of lens 162 andconductive pad 194 (FIG. 6).

As previously noted, and referring also to FIG. 6, piezoelectric filmelement 170 includes first and second electrode layers 176 and 178formed respectively on top and bottom surfaces 172 and 174 ofpiezoelectric film element 170. During assembly, piezoelectric filmelement 170 is adhesively secured to domed surface 164 such that secondelectrode layer 178 and conductive layer 163 are adjacent to and inelectrical contact with each other, as best shown in FIG. 6. Preferably,piezoelectric film element 170 is secured to domed surface 164 withcynoacrylate, although other adhesives, such as silver filled epoxiesare acceptable. When securing piezoelectric film element 170 to domedsurface 164, electrical contact is maintained between second electrodelayer 178 and conductive layer 163 by mechanical contact.

First and second electrode layers 176 and 178 are formed by platingopposing sides of piezoelectric film element 170 with a combination ofplatinum and gold, and conductive layer 163 is formed on domed surface164 from copper. These materials are merely provided as examples ofsuitable coatings, although it should be noted that other similarlyconductive materials can be used in other embodiments. Preferredpiezoelectric film element 170 is a PVDF film as available from Ktech,Inc., 1300 Eubank Blvd., SE, Albuquerque, N.M., 87123-3336. Althoughembodiments are envisioned wherein multiple transducers 160 are used incombination to detect the presence of fluids, preferred embodimentsutilize a single transducer 160 wherein lens 162 and piezoelectric filmelement 170 not only transmit acoustic signals, but also act as anultrasonic receiver for detecting return signals.

Next, insulative disc 186 is secured inside conductive sleeve 180adjacent inwardly depending lip 182. As noted, conductive sleeve 180 iscomprised of stainless steel, and insulative disc 186 is formed of apolymer. Insulative disc 186 is secured to conductive sleeve 180adjacent inwardly depending lip 182 by dimpling conductive sleeve 180such that it grips insulative disk 186. However, various methods, suchas gluing or tacking, are acceptable for use with various otherembodiments. Tab contact 190 is inserted into central aperture 188 ofinsulative disc 186, and conductive pad 194 is secured to the bottomportion of contact tab 190 by a conductive, pressure-sensitive adhesive(not shown). As such, conductive pad 194 extends downwardly from tabcontact 190 and into the interior of conductive sleeve 180.

Conductive sleeve 180 is passed over the upper portion of lens 162 untilbottom edge 184 of conductive sleeve 180 abuts lens annular ledge 168.Once positioned, conductive sleeve 180 is dimpled about lower edge 184so that it grips lens 162. So positioned, conductive pad 194 is inmechanical and electrical contact with first electrode layer 176 ofpiezoelectric film element 170, as best seen in FIG. 6.

Referring to FIG. 7A, an adhesive layer 152 for securing fluid leveldetector 100 to a container wall is secured to bottom surface 122 ofbottom housing 110. Preferably, adhesive layer 152 comprises a layer ofdouble-sided tape having a pressure-sensitive adhesive on both sides.Double-sided tape layer 152 has the same diameter as bottom surface 122of base 118 and has an aperture 152 formed at its center. Aperture 152 ahas a diameter at least equal to that of aperture 120 (FIGS. 7A and 7B).After securing double-sided tape layer 152 to bottom surface 122, anon-stick, peel-away film is adhered to the side of the double-sidedtape layer 152 opposite to that which is secured to bottom surface 122.The peel-away film (not shown) is a solid piece of film similar to thosetypically found on stickers and double-sided tape, that is not removeduntil fluid level detector 100 is to be installed. As such, thepeel-away film inhibits debris, dust, and moisture from entering thehousing by way of aperture 120 prior to the use of fluid level detector100.

A coupling layer 156 is adhered to contact face 166 (FIG. 5A) of lens162. As shown, the diameter of coupling layer 156 is substantiallysimilar to the diameter of contact face 166 and slightly less than thediameter of aperture 152 a formed in the double-sided tape layer 152, asbest seen in FIG. 3. Desirable materials for coupling layer 156 arecapable of being formed in thin sections to minimize acoustic losses,able to conform to surface irregularities, exhibit low acoustic loss,have acoustic impedances that closely match those of most polymers, andare non-aqueous in nature to facilitate extended periods of use.Examples of suitable coupling materials include: urethane, neoprene,thixotropic glycerin, and high-temperature grease, although it should beappreciated that other suitable materials can be utilized.

Next, transducer 160 is slidably received in central bore 114 of bottomhousing 110. Inward motion of transducer 160 is limited by the peel-awaysurface (not shown), such that coupling layer 156 lies in the same plainas double-sided tape layer 152. Note, the greatest outside diameter oftransducer 160 is sized such that transducer 160 readily slides withincentral bore 114 (FIG. 1).

Referring to FIGS. 1, 7A and 7B, after positioning transducer 160 incentral bore 114, top housing 130 is secured to bottom housing 110. Tophousing 130 includes a pair of electrical contacts secured therein. Forease of description, only first electrical contact 196 is shown.Preferably, first electrical contact 196 includes a base portion 196 a,a male electrode 196 b, and a spring 196 c. Base portion 196 a issecurely held by portions of top housing 130. Male electrode 196 bextends from base portion 196 a and into wiring harness receptacle 134of top housing 130. Spring 196 c extends from base portion 196 a andmakes contact with tab contact 190 of transducer 160. Spring 196 c canbe of any suitable configuration, such as a leaf spring or coil spring,that biases the transducer into operative contact with the container.Although spring 196 c can be either metallic or non-metallic, preferredembodiments include metal springs such that the spring itself is anelectrically conductive element.

The biasing element of the second electrical contact (not shown) dependsinwardly from top housing 130 and makes mechanical and electricalcontact with conductive sleeve 180 of transducer 160. The secondelectrical contact also has a male electrode extending outwardly intowiring harness receptacle 134. Engagement of annular groove 133 byannular lip 117 maintains bottom housing 110 and top housing 130 in theassembled position. O-ring 150 is disposed in annular groove 116 betweenbottom and top housings 110 and 130 and helps maintain the structuralintegrity of fluid level detector 100.

Operation

Prior to use, and still referring to FIG. 7A and 7B, fluid leveldetector 100 is first applied to a wall 104 of the container containingthe liquid to be monitored. Preferred embodiments of the present fluidlevel detector 100 are used to monitor fluid levels in polymercontainers or containers constructed of other similar materials havingmaximum wall thicknesses of approximately 0.150 inches (″). For suchcontainers, preferred embodiments of fluid detector 100 includepiezoelectric film elements 170 measuring approximately 0.230″ long,0.125″ wide, and 0.004″ thick, and constructed of PVDF. However, thesedimensions can be varied for container walls of varying thicknesses.

As is known in the art, materials attenuate acoustic energy as theenergy passes there through. Moreover, acoustic energy at higherfrequencies is attenuated over shorter distances within a given materialthan is acoustic energy at lower frequencies. As noted above, preferredembodiments of fluid level detector 100 include a piezoelectric filmelement 170 composed of a PVDF, which has a relatively high naturaloperating frequency of approximately 10,000,000 Hz (10 MHz). Thus,preferred embodiments of fluid detector 100 are typically used onpolymer containers having maximum wall thicknesses of up to 0.150″ sothat adequate return signals exist for fluid detection, as discussedhereafter.

Increasing the size (length by width) of piezoelectric film element 170permits fluid level detector 100 to be used with greater wallthicknesses since a greater amount of wall material is required toattenuate the larger amount of acoustic energy that is generated. Thisrequires a corresponding increase in the domed top surface of lens 162to accommodate the larger piezoelectric film and focus its resultingacoustic signals. The amount of acoustic energy generated by thetransducer can also be increased while maintaining the size of both thedomed top surface of lens 162 and piezoelectric film element 170 by“stacking” multiple film elements. By stacking multiple piezoelectricfilm elements atop each other and electrically connecting them, eitherin parallel or in series, the amount of acoustic energy generated willbe the cumulative amount of that energy generated by each piezoelectricfilm.

Fluid level detector 100 can also be used with greater wall thicknesseswhen piezoelectric film element 170 is composed of piezoelectricmaterials with lower natural frequencies since the generated acousticenergy travels farther through the same materials than does the highfrequency acoustic energy before being detrimentally attenuated.Moreover, for a given wall thickness, acoustic energy at lowerfrequencies provides larger return signals than does acoustic energy athigher frequencies.

Prior to installing fluid level detector 100 on the container wall, thepoint on container wall 104 corresponding to the desired level ofdetection is determined. The installer then removes the peel-awaysurface (not shown) disposed on the bottom face of double-sided tapelayer 152. With the peel-away surface removed, force exerted ontransducer 160 by springs 196 c (only one is shown) urges transducer 160along central bore 114 such that contact face 166 of lens 162 extendsbeyond double-sided tape layer 152, as shown in FIG. 7A. Outward motionof transducer 160 caused by springs 196 c ceases when annular ledge 168abuts the inwardly depending ledge of bottom housing 110 that definesaperture 120. Note, contact face 166 and coupling layer 156 extendslightly beyond double-sided tape layer 152 such that full contactbetween transducer 160 and the container wall 104 is possible.

Fluid level detector 100 is then pressed firmly against container wall104 at the desired location. Double-sided tape layer 152 secures fluidlevel detector 100 to container wall 104, as shown in FIG. 7B. As fluidlevel detector 100 is pressed against container wall 104, contactbetween container wall 104 and contact face 166, by way of couplinglayer 156, urges transducer 160 back inside central bore 114. Springs196 c maintain pressure on transducer 160, thereby ensuring properpositioning of contact face 166 adjacent the outer surface 104 a ofcontainer wall 104. As such, springs 196 c serve to bias piezoelectricfilm element 170 operatively toward outer surface 104 a of wall 104.

By biasing piezoelectric film element 170 operatively toward wall 104,springs 196c bias the piezoelectric film element 170 either directlyagainst wall 104 or against intermediate components, such as lens 162and coupling layer 156 in the embodiment shown. These components in turncouple the acoustic signal generated by piezoelectric film element 170to wall 104. Although in the latter case springs 196 c still biaspiezoelectric film element 170 in the direction of wall 104, it ispossible that in other arrangements springs 196 c will biaspiezoelectric film element 170 in a direction other than toward wall104, yet still into coupling elements such that the acoustic signalgenerated by piezoelectric film element 170 is nevertheless coupled tothe wall. In such arrangements, springs 196 c are said to biaspiezoelectric film element 170 operatively toward the wall although theydo not bias piezoelectric film element 170 directionally toward thewall.

Wiring harness 140 includes female electrode receptacles 142 (only oneis shown). Wiring harness 140 is slidably received inside wiring harnessreceptacle 134 of top housing 130 such that female electrode receptacles142 are connected with male electrodes 196 b. Electronic signals to andfrom fluid level detector 100 may now be transmitted to the detector asdesired.

In preferred embodiments, the input electrical signal to fluid leveldetector 100 is a ten volt peak-to-peak (ground to +10 volts) pulselasting approximately 50 nanoseconds, or a twenty-four volt (+12 v to−12 v) square wave lasting approximately 100 nanoseconds, (hereafter,“excitation signal”). As shown in FIGS. 7A and 7B, the excitation signalis applied across opposing top and bottom sides 172 and 174 ofpiezoelectric film 170 by way of two independent electrical paths. Inthe illustrated embodiment, the first electrical path is as follows:from first electrical contact 196 to tab contact 190 by way of spring196 c; from tab contact 190 to conductive pad 194; and from conductivepad 194 to first electrode layer 176 formed on top surface 172 ofpiezoelectric film element 170 (as shown in FIG. 6). The second path isas follows: from the second electrical contact (not shown) to conductivesleeve 180 by way of the spring, from conductive sleeve 180 toconductive layer 163 disposed on domed surface 164; and from domedsurface 164 to second electrode layer 178 formed on bottom surface 174of piezoelectric film element 170 (as shown in FIG. 6).

Application of the excitation signal creates vibrations in piezoelectricfilm element 170. PVDF is used for piezoelectric film element 170because of its inherently low acoustic impedance and natural frequencyof approximately 10 MHz. Typical acoustic impedance values for PVDFrange from 2.5×10⁶ to 3.0×10⁶ Rayleighs (2.5 to 3.0 Mrayls), making thepiezoelectric film desirable for transmitting acoustic signals intowalls of similar-impedance polymer containers with minimal losses. Mostpolymers have impedance valves of between about 1.5 and 3.0 Mrayls.

Because piezoelectric film element 170 is secured to domed surface 164,vibrations of piezoelectric film element 170 create pressurefluctuations in the material of lens 162. As previously noted, lens 162is preferably constructed of a polystyrene or other like material suchthat the acoustic impedance of lens 162 will be substantially similar tothat of piezoelectric film element 170 and that of wall 104 of thecontainer. Substantially similar acoustic impedance values for thevarious materials facilitate the passage of acoustic energy as eachtransmitted signal propagates into the adjacent materials. Preferably,acoustic impedance values of the materials used to constructpiezoelectric film element 170, lens 162, and container wall 104 arewithin 2.5 Mrayls of each other. Thus, each acoustic signal generated bythe excitation of piezoelectric film element 170 propagates from onecomponent to the next with acceptable energy loss, insuring effectiveoperation of fluid detector 100.

The generated pressure fluctuations propagate through lens 162 untilthey reach contact face 166. As shown in FIGS. 5A and 5B, in a preferredembodiment, each point along domed surface 164 is equidistant from afocal point (f₁) located on contact face 166 of lens 162, this distancebeing the radius of curvature of the domed surface 164. Lens 162 therebyfocuses a maximum amount of pressure fluctuation at focal point (f₁) asthe acoustic signal travels from domed surface 164 toward contact face166. It should be noted that focal point (f₁) need not be located oncontact face 166 of lens 162. For example, preferred embodiments havefocal points (f₁) located on inner surface 104 b of container wall 104,as shown in FIG. 7B, provided the width of both coupling layer 156 andcontainer wall 104 are known. Similarly, focal points (f₁) may beselected that are located within lens 162, wall 104, or the fluid to bedetected.

The piezoelectric element of the preferred embodiments is apiezoelectric film element 170. As is known in the art, acoustic outputpower of piezoelectric films is generally less than that ofpiezoelectric ceramics in response to comparable input signals. Byfocusing the pressure fluctuations transferred from piezoelectric filmelement 170 to lens 162 at focal point (f₁), however, lens 162 transfersthe relatively lower pressure across the surface of the piezoelectricfilm element to a higher pressure at focal point (f₁). Lens 162 therebydelivers a sufficiently strong acoustic signal to container wall 104 tofacilitate operation of fluid detector 100.

Lens 162 also functions as an acoustic standoff. More specifically, lens162 is dimensioned such that reflected acoustic signals are not receivedat piezoelectric film element 170 until after piezoelectric film element170 has ceased vibrating in response to application of the excitationpulse. Lens 162 facilitates operation of fluid level detector 100 byinsuring that reflected signals for determining the presence or absenceof fluids are not received until after the transmission phase ofpiezoelectric film element 170 has subsided.

In the preferred embodiment, the radius of curvature of domed surface164 of lens 162 is measured from focal point (f₁) on inner surface 104 bof wall 104 (FIG. 7B) and is approximately 0.40″. The preferred radiusof curvature takes into account the height of lens 162 (0.250″ in thepreferred embodiment), the thickness of coupling layer 156, and thethickness of wall 104. Considerations for the height of lens 162 caninclude adequate distance to accomplish acoustic standoff for thefrequency of the acoustic signal being used and potentially the overalldimensions of fluid level detector 100. For example, greater lensheights can be required to provide sufficient acoustic standoff forlower frequency acoustic signals used with containers having greaterwall thicknesses to avoid detrimental attenuation.

Although, ideally, no acoustic impedance mismatch would exist at therespective interfaces between the materials of interfaces piezoelectricfilm element 170, lens 162, and container wall 104, there will normallybe at least slight impedance mismatches. Transmission of the acousticsignal will therefore be affected as it passes from one material to thenext. For example, an impedance mismatch likely exists at the boundaryof contact face 166 and outer surface 104 a of container wall 104, wherelens 162 and wall 104 are operatively coupled to allow transmission ofacoustic signals therebetween. The impedance mismatch between thematerials of lens 162 and container wall 104 causes a portion of theenergy of the acoustic signal to be reflected back through lens 162,eventually reaching piezoelectric film element 170. The first reflectionof acoustic energy causes vibration of piezoelectric film element 170.In response, piezoelectric film element 170 creates an electrical signalacross its electrodes that is sent to an electronics module, asdiscussed hereafter, as a single pulse or echo. That portion of theacoustic signal that is not reflected at contact face 166 of lens 162continues to propagate into the next material layer.

Transmission of acoustic signals from one material to the next isfacilitated when acoustic impedances of the materials are matched andwhen the two abutting surfaces are in full contact. Coupling layer 156is therefore preferably disposed between contact face 166 and outersurface 104 a of container wall 104 and is preferably composed of amaterial that is sufficiently pliant to accommodate surfaceirregularities between contact surface 166 and outer surface 104 b,thereby preventing the formation of air pockets between the abuttingsurfaces that would otherwise degrade propagation of acoustic signals.

The material of coupling layer 156 is also chosen to minimize theeffects of any acoustic impedance mismatch between the materials of lens162 and container wall 104. It is expected that fluid level detector 100will be used to detect fluid levels in containers constructed of variouspolymers having acoustic impedance valves in the range of 1.5 to 3Mrayl. In the event the acoustic impedance of the container wall is notsufficiently matched to the acoustic impedance of lens 162, a couplingmaterial can be used to improve the impedance match. For example, theacoustic impedance of coupling layer 156 is preferably between theacoustic impedance values of lens 162 and the container wall such thatthe acoustic signal encounters the overall mismatch incrementally ratherthan all at once. This facilitates transfer of acoustic energy from lens162 to coupling layer 156 and from coupling layer 156 to the containerwall.

The coupling layer, although chosen to enhance acoustic impedancematching, results in two interfaces at which a slight mismatchnevertheless occurs—between lens 162 and coupling layer 156 and betweencoupling layer 156 and the container wall. The two interfaces result intwo reflected signals when the acoustic signal from the piezoelectricelement passes through the coupling layer to the wall. As indicatedabove, the reflected acoustic energy travels back through lens 162 andeventually causes vibration of piezoelectric film element 170.Preferably, however, coupling layer 156 is sufficiently thin that thesecond reflection, i.e. due to the coupling layer 156\container wall 104interface, arrives at the piezoelectric film at substantially the sametime as does the first reflected signal, i.e. due to the lens162/coupling layer 156 interface, and for purposes of this discussion,the two reflections are considered to be a single reflection. Asdescribed in more detail below, the electronics module is configured todisregard this combined reflection.

The remainder of the acoustic signal that has not been reflected at theabove noted material interfaces propagates into and through containerwall 104 until reaching inner surface 104 b, at which point a thirdreflection of acoustic energy occurs. The amplitude of the reflectedacoustic energy is largely dependent upon the size of the acousticimpedance mismatch that occurs between the material of wall 104 and thematerial disposed in the container opposite fluid level detector 100.

Air has an approximate acoustic impedance of 407 rayls. Most polymershave acoustic impedances of between 1.5 to 3.0 Mrayls. Thus, a largeacoustic impedance mismatch occurs at the inner surface 104 b of wall104 when air is located within the container opposite fluid leveldetector 100. Thus, the overwhelming majority of energy of the acousticsignal will be reflected at inner surface 104 b when the container'sliquid level falls below the position at which detector 100 is attachedto the wall. Water, on the other hand, has an approximate acousticimpedance of 1.48 Mrayls, notably closer to the values of acousticimpedances for most polymers, and the reflected energy from an interfacebetween inner surface 104 b and water is therefore small as compared tothe reflected energy when air is present. Most fluids have acousticimpedance values similar to that of water, meaning they have essentiallythe same effect on the acoustic signal as does water. Therefore, whenwater or other fluid is present opposite fluid level detector 100, theoverwhelming majority of acoustic energy is transmitted from containerwall 104 into that fluid, where it eventually dissipates.

The third reflected signal (i.e. due to the interface of the inner wallsurface and air or liquid) propagates back through container wall 104,coupling layer 156, and lens 162 until it reaches piezoelectric filmelement 170. As before, the third reflected signal causes vibration ofpiezoelectric film element 170, resulting in an electrical signal beingcreated across the film's electrodes and sent to the electronic module.The voltage of the signal created by piezoelectric film element 170 isproportional to the amount of energy reflected at inner surface 104 b ofwall 104. Accordingly, a large voltage signal received at the electronicmodule indicates that air or other gas is present in the container atthe level of fluid level detector 100. Conversely, a small voltagesignal received at the electronic module indicates that a fluid ispresent in the container opposite fluid level detector 100.

Operation of the electronics module will now be discussed with respectto FIGS. 8 through 10. Referring initially to FIG. 8, a dual wire bundle141 output from wiring harness 140 (FIG. 1) includes a ground wire 200and a signal wire 202 that carries both the input electrical signal tothe transducer and the output electrical signal corresponding to thereflected ultrasonic signal. Wire 202 electrically connects through thewiring harness to the spring 196 c (FIG. 7A) that contacts tab contact190, whereas wire 200 electrically connects through the wiring harnessto conductive sleeve 180 (FIG. 7A) by the second spring (not shown).Dual wire bundle 141 extends from fluid level detector 100 to a printedcircuit board remote from the detector and upon which the circuitryshown in FIG. 8 is disposed.

A processor 204 (in a preferred embodiment, a four megahertz single chipmicrocontroller) disposed on the printed circuit board controls anexcitation circuit 206, a detection circuit 208 and a blanking periodgenerator circuit 210 through the output of high or low signals (forexample, +5 volts or ground) on a trace 212. Generally, this systemalternates between an excitation mode, in which excitation circuit 206provides an input electrical signal to detector 100, and a detectionmode, in which detection circuit 208 receives and notifies themicroprocessor of signals corresponding to acoustic echoes from a gasinterface at the inner container wall opposite the detector. In apreferred embodiment, the microprocessor triggers the excitation modeonce per second such that the electronics module checks the output offluid level detector 100 for liquid level approximately once per second,although the timing can vary as desired.

Immediately prior to the excitation mode, the output of microprocessor204 on trace 212 is low such that input 214 to a NAND gate 218 is low.The low signal at 212 maintains an input 216 high through a switch suchas a MOSFET 215. The low signal at 214 results in a low signal on line202 such that no excitation signal is provided to the piezoelectric filmof fluid level detector 100. At the beginning of the excitation mode,however, the microprocessor's output to trace 212 goes high, therebyimmediately bringing input 214 high. Since input 216 is normally high,this causes the output of NAND gate 218 to go low. An inverter 220changes the low signal to high at an input 222 to a comparator 224 thatlevel-shifts the signal to +10 volts at 226. A capacitor 228 AC-couplesthe excitation signal, which passes through a diode 230 to input line202 and, then, to the electrodes driving the piezoelectric film.

Diodes 230 and 232 isolate the return signal from the excitationcircuit. As described in more detail below, the return signal on line202 generated by vibrations of the piezoelectric film in fluid leveldetector 100 are of relatively low power such that the return signal isinsufficient to activate diode 232.

The duration of the high portion of the input electrical signal on line202 is defined by the RC time constant of a resistor 234 and a capacitor236. More specifically, the high signal on trace 212 does notimmediately cause MOSFET 215 to bring input 216 low. Instead, 216 goeslow when capacitor 236 charges sufficiently to activate MOSFET 215. Wheninput 216 goes low, the output of NAND gate 218 goes high, causing theoutputs of inverter 220 and comparator 224 to go low. In the illustratedembodiment, the RC time constant of register 234 and capacitor 236 isapproximately 50 nanoseconds. Thus, the duration of the input electricalsignal pulse output by excitation circuit 206 is approximately 50nanoseconds.

Blanking period generator circuit 210 defines the duration of theexcitation mode. Immediately prior to the excitation mode, when thesignal on trace 212 is low, an input 238 to a NAND gate 240 is low.Thus, an input 242 to a NAND gate 244 is high, and the value of anoutput 246 from NAND gate 244 therefore depends upon the signal at aninput 248. Input 248 is the output of detection circuit 208 and, asdescribed in more detail below, is in either a high or low state,depending upon whether detection circuit 208 has received a sufficientlystrong signal on line 202. When detection circuit 208 detects such asignal, the detection circuit places a high signal on trace 248. Thiscauses NAND gate 244 to transition from high to low at 246, therebynotifying microprocessor 204 that a signal has been received indicatingthat fluid level detector 100 has detected air or other gas on theopposite side of the container wall from the detector.

Accordingly, as long as trace 242 remains high, NAND gate 244 passes thedetection signal from detection circuit 208 to the microprocessor. Thiscondition exists during the detection mode, which is therefore definedby the time period during which either the signal on trace 212 is low oran input at 250 is low.

Again referring to the time immediately prior to the excitation mode,the signal on trace 212 is low. Output 242 of NAND gate 240 is thereforehigh, and NAND gate 244 therefore gates the output of detection circuit208 to microprocessor 204. When microprocessor 204 drives the signal ontrace 212 high, however, input 238 to NAND gate 240 immediately goeshigh. Input 250 to NAND gate 240 is normally high during detection mode,and so NAND gate 240 drives trace 242 to a low signal. This causesoutput 246 of NAND gate 244 to be high regardless of the signal fromdetection circuit 208 on trace 248. Accordingly, changes on line 202caused by return signals detected by fluid level detector 100 have noeffect on output 246, and microprocessor 204 therefore does not receivesignals on 246 indicating that such return signals have occurred. Inother words, the transition to the high signal on trace 212 starts aperiod during which blanking period generator 210 blocks detectioncircuit 208 from reporting detection of a return acoustic signal byfluid level detector 100. This condition is the excitation mode.

The duration of the excitation mode is defined by the RC time constantof a resistor 252 and a capacitor 254. A MOSFET 256 normally maintainsthe signal on trace 250 high when the signal on 212 is low. When 212goes high, the RC network 252/254 prevents the new high signal fromimmediately driving the signal on trace 250 low. When capacitor 254eventually charges sufficiently to cause MOSFET 256 to drive the signalon trace 250 low, the low signal causes NAND gate 240 to drive thesignal on trace 242 high regardless of the high signal on 238. Thus,NAND gate 244 again passes the output of detection circuit 208 tomicroprocessor 204, and the system has returned to detection mode.Microprocessor 204 thereafter drives the signal on trace 212 low,thereby resetting excitation circuit 206 prior to triggering the nextexcitation mode on the one second interval.

In a preferred embodiment, the RC time constant of resistor 252 andcapacitor 254 defines the duration of the excitation mode to 5.0×10⁻⁶seconds (5 ms). This period may vary as desired, however, for exampledepending upon characteristics of fluid level detector 100 and thetiming of signals it is likely to detect. For example, and referringalso to FIG. 10A, the excitation period (indicated at 258), andtherefore the RC time constant of resistor 252 and capacitor 254, shouldbe sufficiently long so that blanking period generator circuit 210blocks the responses of detector circuit 208 to both the inputelectrical signal generated by excitation circuit 206 and to signalsreturned on line 202 as a result of ringing of the piezoelectric filmfollowing the excitation signal.

As described above, the input electrical signal from excitation circuit206 is a 10 volt pulse lasting approximately 50 nanoseconds. Detectioncircuit 206 detects this relatively large signal as it is being outputonto line 202. Furthermore, the piezoelectric film in fluid leveldetector 100 vibrates for some period of time after the end of the 50nanosecond pulse. This ringing of the film creates a signal across thefilm's electrodes that is returned to the detection circuit over line202. Thus, during excitation mode, detection circuit 206 sees arelatively large signal, indicated at 260 in FIG. 10A, that wouldotherwise cause the detection circuit to incorrectly send a signal tomicroprocessor 204 indicating an acoustic echo had been received.Because blanking period generator 210 maintains a low signal on trace242 during the excitation mode, however, NAND gate 244 does not gatethis signal to the microprocessor, which therefore sees no false echoreport during this period, as indicated at 262 in FIG. 10B. To assurethat detection circuit 208 does not report a false echo, the RC timeconstant defined by resistor 252 and capacitor 254 should be establishedso that blanking period generator 210 blocks signals detected bydetection circuit 208 for a period longer than the time during whichsignals resulting directly from the input electrical signal (i.e. notfrom an acoustic echo following the input electrical signal) areexpected to be sufficiently high that detector circuit 208 wouldotherwise incorrectly provide a signal to microprocessor 204 indicatingan acoustic echo had been received.

Detection circuit 208 is comprised of a pair of amplifier stages 264 and266, an AC coupling capacitor 268, and a comparator 270. Because signalsgenerated by the piezoelectric film in fluid level detector 100 are ofrelatively low power, for example on the order of 1 to 2 millivolts,amplifier stages 264 and 266 apply an approximately four hundred timesgain to the signal received from fluid level detector 100 over line 202.Comparator 270 then compares the amplified signal to a predeterminedvoltage level defined by divider resistors 272 and 274 at 276. Thevoltage level at 276 is preferably set so that signals generated byacoustic echoes from the transducer and the outer wall of the containerare ignored, while the stronger signals resulting from the container'sinner wall surface and air trigger a change in the detector circuit'soutput.

As described above, the acoustic echo from the interface between thelens and coupling material, and between the coupling material and thecontainer wall outer surface, is weaker than an acoustic echo resultingfrom an air interface with the container inner wall surface. The firstecho therefore results in weaker vibrations in the piezoelectric filmthan does the second echo, and the first echo therefore generates alower voltage signal on line 202. Accordingly, the first echo results inan amplified signal at the input 278 to comparator 270, indicated at 280in FIG. 10B, having a lower voltage level than an amplified signal,indicated at 282, that results from an acoustic echo from the airinterface. The voltage level, indicated at 284, defined by the divideris set higher than the expected level of the first amplified signal (andalso higher than the expected level of an amplified signal resultingfrom an echo from a liquid interface at the container's inner wallsurface) but less than the expected level of the second amplifiedsignal. Accordingly, comparator 270 remains low upon receipt of a signalresulting from an acoustic echo from the container wall's outer surface(or from a liquid interface at the container wall's inner surface) butoutputs a high signal on trace 248 upon receipt of a signalcorresponding to an acoustic echo from the interface between thecontainer wall's inner surface and air. Because the electronic module isnow in detection mode, the signal on trace 242 to NAND gate 244 is high.Thus, the transition of the signal on trace 248 from low to high uponreceipt of an acoustic echo from an air interface drives the outputsignal from NAND gate 244 on trace 246 from high to low. During thedetection mode, this transition notifies microprocessor 204 thatdetector 100 has detected a condition at which fluid level inside thecontainer has fallen below the level of the detector. Microprocessor 204then outputs a signal indicating this condition on a line 286 to anoutput circuit 288 that drives a notification device such as a lamp,audible device or other peripheral device. Alternatively, oradditionally, microprocessor 204 can communicate with a remote processorthrough an RS-232 circuit 290.

In one preferred embodiment, microprocessor 204 repeatedly checks thesignal on trace 246 and does not change the state of its output untildetecting a change on trace 246 at five consecutive reads. This inhibitsfalse responses due to jitter in the digital circuitry.

In a still further preferred embodiment, and referring to FIGS. 8 and 9,excitation circuit 206 is replaced by an excitation circuit 292 toprovide a square wave electrical signal rather than a pulse. Excitationcircuit 292 is comprised of an H-bridge circuit 294 controlled by atiming circuit 296. H-bridge circuit 294 applies a square wave to fluidlevel detector 100 across lines 200 and 202 that varies between −12volts and +12 volts.

Prior to the excitation mode, the signal on trace 212 is low. Thiscauses timing circuit 296 to de-activate the H-bridge circuit such thatno input electrical signal is provided to the detector. Whenmicroprocessor 204 applies a high signal to trace 212 at the beginningof the excitation mode, however, the high signal immediately turns onswitches 298 and 300 (which may be, for example, high speed MOSFET's orbipolar transistors), thereby applying a 12 volt signal from the powersource to the H-bridge. An approximate two nanosecond delay caused by aresistor 302 and switch 304 allows the H-bridge circuit to power upthrough switches 298 and 300 before the occurrence of subsequenttransitions.

The activation of switch 304 turns on switches 306 and 308 (again which,for example, may be MOSFET's or bipolar transistors) in the H-bridge,thereby applying a +12 volt signal to the piezoelectric film acrosslines 200 and 202. Meanwhile, capacitors 310 and 312 charge throughresistors 314 and 316, respectively. Capacitor 312 charges first,thereby turning on switch 318. This grounds the gate of switch 304 andthereby turns off switches 306 and 308.

The time constant defined by resistor 314 and capacitor 310(approximately 50 nanoseconds) is such that, at this point, a switch 320turns on, thereby turning on switches 322 and 324 through switches 326and 328. This applies a −12 volt portion of the square wave across lines200 and 202. Microprocessor 204 then (approximately 50 nanosecondslater) drives the signal on trace 212 low, thereby deactivating theH-bridge circuit.

In the configuration of the blanking period generator circuit shown inFIG. 8, the low signal on trace 212 also returns the electronic moduleto receive mode. In the event that ringing in the piezoelectric filmfollowing the end of the square wave does not result in a return signalof sufficient magnitude to require blocking of the detection circuit bythe blanking period generator, or if the microprocessor is programmed toignore such a response, this is acceptable. In the event, however, thatit is desired to block the detection for a certain period of time inorder to block signals resulting from ringing in the piezoelectric filmfollowing the input electrical signal, blanking period generator circuit210 is preferably modified so that the output of NAND gate 244 remainshigh for a sufficiently long period after the end of the inputelectrical signal.

While one or more preferred embodiments of the fluid level detector havebeen described above, it should be understood that any and allequivalent realizations of the fluid level detector are included withinthe scope and spirit thereof. For example, the piezoelectric film andlens can be replaced by one or more piezoelectric ceramic elements.Because ceramic elements produce stronger electrical signals,amplification in the electronics module can be reduced or eliminated.Thus, the depicted embodiments are presented by way of example only andare not intended as limitations on the fluid level detector. It shouldbe understood that aspects of the various one or more embodiments may beinterchanged either in whole or in part. Therefore, it is contemplatedthat any and all such embodiments are included in the present disclosureas may fall within the literal or equivalent scope of the appendedclaims.

1. A transducer for use in a fluid detector for determining a presenceof a fluid within a container having a wall with an outer surface and aninner surface, the transducer comprising: a piezoelectric element thatoutputs a first ultrasonic signal in response to an input electricalsignal; and a lens with an upper portion, a lower portion, and a focalpoint, the upper portion including a curved surface with a convex side,wherein the piezoelectric element is coupled to the upper portion of thelens so that, when the lower portion of the lens is disposed adjacentthe outer surface of the wall at a predetermined position, the lens isintermediate the piezoelectric element and the wall and the lens focusesthe first ultrasonic signal toward the focal point of the lens andtoward the wall, and wherein, when the transducer receives a secondultrasonic signal from the wall that results from the first ultrasonicsignal and that is affected in a predetermined manner by presence orabsence of fluid at the inner surface of the wall, the ultrasonictransducer generates an output electrical signal corresponding to thesecond ultrasonic signal.
 2. The transducer as in claim 1, wherein thepiezoelectric element comprises a piezoelectric film.
 3. The transduceras in claim 1, wherein the upper portion of the lens further comprises adomed surface, the piezoelectric film being operatively coupled to theconvex side.
 4. The transducer as in claim 2, wherein the piezoelectricfilm comprises a polyvinylidene fluoride copolymer.
 5. A fluid detectorfor determining a presence of a fluid within a container having a wallwith an outer surface and an inner surface, the fluid detectorcomprising: an ultrasonic transducer including: a piezoelectric elementthat outputs a first ultrasonic signal in response to an inputelectrical signal; and a lens with an upper portion, a lower portion,and a focal point, wherein the piezoelectric element is coupled to theupper portion of the lens so that, when the lens is disposed adjacentthe outer surface of the wall, the lens focuses the first ultrasonicsignal toward the focal point of the lens and toward the wall so thatthe first ultrasonic signal enters the wall; and wherein the ultrasonictransducer, when disposed in a predetermined position adjacent the outersurface of the wall, receives a second ultrasonic signal from the wallthat results from the first ultrasonic signal and that is affected in apredetermined manner by presence or absence of fluid at the innersurface of the wall, and the ultrasonic transducer generates an outputelectrical signal corresponding to the second ultrasonic signal.
 6. Thefluid detector as in claim 5, wherein the piezoelectric elementcomprises a piezoelectric film.
 7. The fluid detector as in claim 6,further comprising: a housing including a central bore, a top portionand a base defining an aperture that communicates with the central bore,wherein the lens and the piezoelectric element are disposed within thecentral bore, and the base is configured to be secured to the outersurface of the wall.
 8. The fluid detector as in claim 7, wherein thepiezoelectric film comprises a polyvinylidene fluoride copolymer.